This is the first of several pages explaining and diagramming the eight stages of meiosis. Meiosis is the two-stage form of cell division that produces four haploid cells from a single diploid cell. This process divides a single nucleus (see figure right), containing two sets of replicated chromosomes (i.e., ones composed of two sister chromatids), into four nuclei, each containing a single set of unreplicated chromosomes (but see note below).
Now, if you already know the answers to the two questions in the green box below, go to the next page for a description of prophase I, the first stage of meiosis.
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TWO COMMON QUESTIONS:
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When does meiosis occur? Meiosis occurs only in eukaryotes. In animals, it occurs during the production of gametes. In plants, it takes place when spores are produced (plant gametes are produced by mitosis). Meiosis does not occur in prokaryotes (i.e., archaea and bacteria). They reproduce by binary fission.
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Why is meiosis necessary? Meiosis has to take place at some point in a sexual reproductive cycle because it halves the chromosome number, which compensates for the doubling of the chromosome number caused by fertilization.
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Note: In the case of oogenesis, a single egg cell and three polar bodies are produced by meiosis.
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Eukaryotic Nucleus:
(click on words in figure to see their definitions)
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